Lee Jae-myung, the leader of South Korea’s main opposition party, was stabbed in the neck on Tuesday, according to the police and live-streamed TV footage.

Mr. Lee, the leader of the Democratic Party, was visiting the southern port city of Busan when an unidentified man stabbed him in the neck with a knifelike weapon, according to the footage. Mr. Lee was leaving through a crowd of reporters and supporters when the attack happened.

The police in Busan said the attacker had been detained, but they did not provide any details about Mr. Lee’s condition or the motives of the attacker.

This is a developing story.